The present invention relates to an actuating unit.
An actuating unit for a wheel brake of a motor vehicle is described in German Published Patent Application No. 195 27 936, where a variable-speed electric motor drives a gear wheel pump, which draws in pressurized media from a reservoir and delivers it to a cylinder-piston unit whose pistons are able to press a brake lining against a friction element connected to a vehicle wheel. Provided on the delivery side of the pump is a throttle which is used to control a connection to the reservoir. The pump drive and the throttle are controlled by an electronic brake control to effect pressure build-up, pressure maintenance and pressure reduction in the cylinder-piston unit.
The known actuating unit is disadvantageous insofar as a pressurized-media-conducting connection to the reservoir is present whenever the brakes are actuated. Thus, the actuating unit is not a closed hydraulic system, which is a significant safety shortcoming. Moreover, relatively large dimensions are required for the reservoir in order to provide adequate pressurized media-volume given a seriously worn brake lining.
In contrast, the actuating unit according to the present invention has the advantage of an increased operational reliability due to the fact that it is a closed hydraulic system. When a braking action occurs, the same pressurized-media volumes are constantly exchanged between the cylinder chamber of the cylinder-piston unit on the brake-lining side and the cylinder chamber facing away from the brake lining. In this manner, the changed piston position is compensated for even when the brake linings are worn. If the hydraulic system is completely sealed, it will not be necessary to replenish the pressurized media. This simplifies servicing the actuating unit, of which a passenger car, for instance, has four. In addition, the actuating unit may be installed in the vehicle already filled with pressurized media.